DC-DC converters are used in various fields. For example, a mobile electronic device uses a battery, such as a battery cell, as a power source, and battery power is discharged with time in accordance with power consumption due to operation of the device, so that the output voltage of the battery reduces. In order to maintain the voltage value of the power source of the device at a certain level relative to such change of the battery voltage with time, the voltage of a power supply source is stabilized by a DC-DC converter.
In a DC-DC converter used in a mobile electronic device, it is desired that power loss is minimized in order to extend an operation time of the device before a battery is to be recharged or replaced, and specifically, highly efficient power conversion is desired. In general, such a DC-DC converter is realized by a synchronous rectification converter.
A synchronous rectification converter includes an inductor (a choke coil), a first (main) switching element, a second (synchronous rectification) switching element, a capacitive element (capacitor), two input terminals, two output terminals, and a control circuit. A direct-current voltage is supplied to the two input terminals from an input power source. The two output terminals are coupled to a load RL. A second one of the two output terminals is coupled to a second one of the two input terminals. The first switching element is coupled between one end of the inductor and a first one of the input terminals. The second switching element is coupled between the one end of the inductor and the second one of the input terminals. The capacitive element is coupled between the other end of the inductor and the second one of the input terminals. The first switching element is made conductive to supply a current to the inductor, and the first switching element is turned off to stop current supply to the inductor. The inductor operates so as to continuously cause a current to flow, and thus, when the second switching element is made conductive, a current is supplied from the second switching element. Due to this operation, the capacitive element is charged via the inductor.
As described above, the first switching element and the second switching element are alternately made conductive (turned on) on a predetermined cycle, and a charge voltage of the capacitive element changes in accordance with a time which it takes to make the first switching element and the second switching element conductive. Such control is referred to as a continuous mode, and control which causes a conduction time to change is referred to as pulse width modulation (PWM) control. When a load is large and an output current (a power supply amount) is large, the charge voltage of the capacitive element reduces and, when the load is small and the output current (the power supply amount) is small, the charge voltage of the capacitive element increases. Therefore, the charge voltage of the capacitive element is detected and the time which it takes to make the first switching element and the second switching element conductive is controlled in accordance with the charge voltage of the capacitive element.
Each of the first switching element and the second switching element is realized by a MOSFET, or the like, has a smaller conduction loss than that of a diode, or the like, which is used as a rectifying element in an asynchronous converter, and is capable of performing accurate control in accordance with the state of a power source, thus allowing reduction in power loss.
It is desired to increase the efficiency of a synchronous rectification converter used for the above-described device in a wide load region ranging from a heavy load with a large power supply amount to a light load with a small power supply amount. In this case, there is a problem in which, at light load, a current flows back in the first and second switching elements, and therefore, power conversion efficiency greatly reduces at light load, as compared to the power conversion efficiency at heavy load.
In order to solve this problem, a technique was proposed in which, when it is detected that a current flowing in the second switching element is zero, that is, when current inversion is detected, control is performed such that the second switching element is turned off, thereby not allowing reverse flow of a current. This control method is referred to as diode emulation (discontinuous mode control), and the efficiency at light load is improved by discontinuous mode control.
As another point, in a synchronous rectification converter, when a load abruptly changes and thus an output voltage (the voltage of a capacitive element) changes, it is desired that the output voltage returns to a predetermined output voltage in a short time. This is referred to load abrupt change responsiveness. In discontinuous mode control, a zero-cross frequency of a gain of a converter reduces and responsiveness is deteriorated, and thus, restoration of the output voltage after abrupt change of a load is delayed. That is, there is a problem in which, in discontinuous mode control, load abrupt change responsiveness is low.
In order to solve the above-described problems, a technique was proposed in which the second switching element is not turned off immediately after current inversion is detected, but the second switching element is turned off in a delayed state after current inversion is detected. Thus, inversion current is caused to flow in the inductor, thereby reducing an excessive flow of energy of the inductor to the load at light load. Using this control method, an operation between a continuous mode and a discontinuous mode is performed, so that energy accumulated in the inductor is allowed to be discharged relatively early, and therefore, responsiveness is improved. However, there is a problem in which efficiency reduces at light load.
The following is a reference document.
[Document 1] Japanese Laid-open Patent Publication No. 2006-014482.